1. Field of the Invention
This invention relates to a digital multiplex transmission system by which a signal such as an audio signal or a video signal can be transmitted fully digitally with an arbitrary redundancy selectively set in accordance with an object of a user and is suitably applied particularly to transmission which makes use of an artificial satellite.
2. Description of the Related Art
In recent years, broadcasting communications which make use of an artificial satellite have been realized. However, a communication system which allows full digital transmission has not been realized as yet except broadcasting communications for an audio signal. Transmission of digital audio data and digital data employed in television broadcasting by broadcasting communications is illustrated in FIG. 13. In particular, FIG. 13 illustrates a two-dimensional frame construction for transmission of digital audio data and digital data. Referring to FIG. 13, voices 1 to 4 are audio data which are compressed in 10 bits for each one sample; a range bit is a bit indicating a range of one bit representing a quantization bit range; data are digital data constituted from 15 bits for each one sample; and a correction code is an error correction code constituted from a 7-bit BCH code which allows correction of an error of 1 bit and detection of an error of 2 bits.
The frame shown in FIG. 13 includes 63 bits in a horizontal row and 32 bits in a vertical column so that the one frame is constituted from a total of 2,048 bits. Of the 63 bits in a horizontal row of the frame, one bit is allocated to the range bit; ten bits are allocated to one sample of each of the voices 1 to 4 so that a total of 40 bits are allocated to the voices 1 to 4; fifteen bits are allocated to one sample of the digital data; and seven bits are allocated to the error correction code.
Meanwhile, in a vertical column, digital audio data and digital data of the voices 1 to 4 for 32 samples are arranged vertically so that each vertical column includes a total of 32 bits. The error correction code is a code for correcting an error of a total of 56 bits of the range bit, the voices 1 to 4 and the digital data which are included in a horizontal row of the frame. The frame is scanned vertically as seen from FIG. 13 to effect interleaving to send out resultant data into a transmission line. The technique described above regarding satellite broadcasting is disclosed in EP 0,548,844 A2 while the technique regarding error correction is disclosed in EP 0,548,887 A2.
In such a multiplex transmission system which employs an artificial satellite as described above, the transmission output from the artificial satellite is so limited that, when the weather is bad such as rainy or when a reception antenna is small, ordinary transmission cannot be achieved and consequently a reception picture may be disturbed. In such an instance, if the redundancy is increased to strengthen the error correction codes instead of degrading the quality of the image, an image of a somewhat higher quality can be received regularly without varying the transmission rate. On the other hand, when the reception antenna is large or the weather is fine, if the redundancy is decreased to raise the quality of the picture, a superior image can be received without varying the transmission rate.
Further, when the allocated band width within a transmission band held by the satellite is changed, transmission cannot be performed without varying the transmission rate or efficient transmission cannot be performed. However, in such a conventional multiplex transmission system as described above, since the frame construction or the construction of an error correction code is fixed, there is a problem in that the quality of the image or the construction of the error correction code cannot be altered and also the transmission rate cannot be changed.